In laser scanning optics, a polygonal mirror or the like is used for scanning the laser beam. High speed rotation of the polygonal mirror causes the tilting of the deflecting or reflecting surface thereof and results in irregularity in the pitch of the scanning lines on a scanned surface. For eliminating the irregularity mentioned above, U.S. Pat. No. 3,750,189 has proposed an optical system for a laser printer in which the laser beam to be incident on the deflecting surface is formed in a line image and is restored after the deflection.
In the meantime, a laser COM system, in which microfilms are exposed by a laser beam modulated in accordance with data from a computer through an optical system including a polygonal mirror and f.theta. lens system, requires high dot density such as 9600 dots/inch for the recording on the microfilm of 15 mm in width. The high dot density requirement leads in an optical system with a short focal length of about 50 mm. The conventional optical systems with the tilting correction function for use in laser printers can not be adapted to the laser COM system since these optical systems have a long focal length, for example, 200 mm to 300 mm.
The high dot density requirement may be fulfilled by an optical system having a scanning lens system (f.theta. lens) and a reduction lens system which projects reduced images. However, this optical system has a drawback of large size.
Furthermore, a laser COM system for a high dot density recording requires a large aperture ratio for the optical system used therein to the extent of FNo.=3.
Japanese Laid-Open Patent Application No. 57-105715 discloses a telecentric f.theta. lens system with FNo.=3 for use in a laser COM system. However, the disclosed telecentric f.theta. lens system has no tilting correction function and can not be combined with the conventional tilting correction optics, because no space extends between the deflecting surface and the recording surface. The telecentric lens system, in which parallel light rays are emitted if the light rays come into the lens system from the focal point thereof, is thought to be suitable for the laser COM.